The objective of this research is to establish the theoretical and practical foundation for a glucose sensor suitable for implantation as the measuring component of an artificial endocrine pancreas (glucose-activated insulin pump). The proposed research is based on the assumption that automatic control of blood glucose in the diabetic would do much to ameliorate the consequences of this disease. It may be particularly effective in the control of juvenile diabetes. Such a glucose sensor must have long term (months or years) function, good stability and the capability of in situ calibration. Modern enzyme electrodes offer great promise but require specific designing to meet these requirements. Specifically, a diffusion-limited glucose oxidase-coated peroxide-sensing platinum anode may prove to fit the bill. Glucose and oxygen diffusing from plasma or tissue water are converted to peroxide which generates a current proportional to the glucose molecules oxidized. The membranes which support the enzyme also serve to exclude all interfering substances such as ascorbic and uric acids as well as drugs such as acetaminophen. The enzyme is stabilized by covalent immobilization. Completed sensors, about the size of a lentil, will be implanted subcutaneously, intraperitoneally, on the liver, and in the right heart (as with pacemaker leads) in cats. The currents (nanoampere range) will be recorded over long periods of time and related to the blood glucose levels as measured from micro blood samples taken before, during and after glucose infusions. The knowledge gained from in vitro and in situ tests will be used to design a glucose sensor to be tested with an insulin pump in cats with streptozotocin-induced diabetes.